"Over the past 12 months, we have gone from defending our bullish stance with price targets at one point well above both market levels and consensus expectations to reconciling near-term caution," he wrote in a new note to clients.

Indeed, when the S&P started the year at 1,426, Belski looked optimistic. With the S&P at 1,665 today, he's now looking a bit bearish.

"[W]e are becoming increasingly alarmed that very little analysis, process, and common sense are being utilized by many investors at current levels," he wrote in a new note to clients.

"From our perspective, the main reason for this abandonment is the thirst for performance and the instant gratification of portfolio gains – particularly considering the degree of fund manager underperformance over the past several years," he added. "After all, stronger periods of bull markets often feed on optimism and “the trend” is rarely not your friend. Consequently, fundamentals have a knack of taking the back seat when the pendulum swings from doubt to complacency, which clearly appears to be the case in the current environment."

Belski included this chart to show how this current bull market stands out in the history books.

BMO Capital

The low volatility of this rally has Belski increasingly concerned that a sell-off, should it come, could be a violent one (emphasis added):

Therein lies the biggest issue, in our view – we believe many investors are forgetting about the potential pain and downside risk associated with stock prices disconnecting from fundamentals. Keep in mind, the market rarely moves in a linear direction for long periods of time. In fact, sharp linear moves in one direction are typically met with even sharper linear moves in the opposite direction (once an inflection point is triggered). Our fear is that fundamentals will not catch up fast enough to support current prices and ultimately trigger a downside inflection point (let’s hope we are wrong). To that end, this week we attempt to answer and provide context for some of the more common questions/rebuttals we are receiving from clients with regard to our market targets and sector opinions.